When you say "figure" do you mean the figure object (the window on the screen) or the axes contained in the figure? I've seen people use that word in both meanings.
If you want to maximize the figure object (the window on the screen) and you're using release R2018a or later, use the WindowState property of the figure object.
If you want to make the axes span the entirety of the figure window, do as Adam Danz suggested.
If you want the axes to span the entirety of a maximized figure object, combine Adam's suggestion and mine. Maximize the figure then make the axes with 'Units', 'normalized' and 'Position', [0 0 1 1].